The present invention relates to devices capable of dispensing minute droplets or dots of viscous material such as adhesives at very high rates, such as twenty dots per second. More particularly the present invention relates to a disposable nozzle assembly which is connected to a syringe filled with viscous material in such high speed dispensing devices.
The dispensing of adhesives quickly and reliably in the manufacture of circuit boards on which components are surface mounted is a difficult task. There are inherent speed limitations associated with rotary positive displacement valves, pneumatically actuated syringes and mechanically actuated pinch tubes used in conventional viscous material dispensers. Warped boards, air in the syringe, and stringing of the surface mount adhesive cause inconsistent dispensing and create the need for inspection and rework. As a result, the adhesive dispenser often becomes the bottleneck in the pick-and-place line.
Suppliers in the fluid dispensing industry have been able to make steady incremental improvements in dispensing speed over the past several years to achieve eights dots per second. However, inconsistencies and the need for inspection and rework were not adequately addressed until ASYMTEK of Carlsbad, Calif. developed the Dispense Jet (Trademark) apparatus disclosed in allowed U.S. patent application Ser. No. 07/978,783 filed Nov. 19, 1992, now U.S. Pat. No. 5,320,250, and entitled METHOD AND APPARATUS FOR RAPID DISPENSING OF MINUTE QUANTITIES OF VISCOUS MATERIALS. The Dispense Jet apparatus uses a nozzle and syringe in combination with a feed chamber. The nozzle is impacted by a solenoid actuated hammer to rapidly reduce the volume of the feed chamber. This causes a jet of viscous material to be ejected from the nozzle and to break away from the nozzle as a result of its own forward momentum. With this new system, it is possible to dispense 72,000 dots per hour from a single head "on the fly" as it passes laterally over a PC board. Adhesive stringing is eliminated with this approach because it does not require wetting of the workpiece surface as is the case with traditional syringe dispenser. The dots generated by the Dispense Jet apparatus have a consistent size regardless of height variations in the board due to warpage.
When traditional viscous material dispensers have been used to apply drops of adhesive and other viscous materials it has been necessary to periodically replace an empty syringe. Such syringes are standard in the industry and employ a luerlock type fitting. An air bubble frequently ends up being entrapped during syringe replacement. This causes problems for standard needle type dispensers that employ positive displacement valves and pinch tubes, and for the newer DispenseJet apparatus. These problems ultimately manifest themselves in the form of missing or variable size dots. The reason that such air bubbles form is that the sealing surfaces on the syringe and the fitting mate before the air trapped between them has a chance to be evacuated from the fluid path.
Standard viscous material dispensers have components, such as rotary positive displacement valves, that must be periodically cleaned. Such cleaning is not only required for regular maintenance, but is necessary when a switch is made in the type of viscous material being dispensed, e.g. from adhesive to potting compound. It is tedious to perform such cleaning, and the dispensing equipment experiences down time. New legislation banning the use of dangerous solvents and CFCs adds to the need to eliminate cleaning of dispenser components with solvents. The dispensing apparatus disclosed in the aforementioned patent application has a nozzle and feed chamber that need not be cleaned but has been designed to have all wetted parts be disposable.